If the universe expanding, and why Andromeda is heading to our galaxy?
- ZheiaLv 62 months ago
It's not heading towards us, but it is moving away slowly and we are moving faster.
- Anonymous2 months ago
You answered your own question. The universe is expanding, not necessarily moving away. Space is not expanding & gravitation is still a force. But, don't worry. By the time (a billion years) Andromeda gets here, you'll be very old.
- 2 months ago
We see galactic *clusters* moving away from each other... but, there are galaxies within those clusters that are gravitationally bound to each other. The Milky Way is part of the "Local Group" of galaxies - about 50 of them, where Andromeda is the largest. Our local group orbit a revolve about a common center of gravity, and - it just so happens that our two galaxies are on a collision course with each other... they'll merge at about the time our sun begins to die.
- FredLv 52 months ago
What part of 'random expansion' do you fail understanding?
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- Anonymous2 months ago
Well there is so many galaxies I don't think anyone would even notice.
- nineteenthlyLv 72 months ago
Because space expands as a whole. Why do objects move towards each other on this planet?
- CliveLv 72 months ago
The fact that the universe as a whole is expanding doesn't prevent individual objects in it heading in different directions.
- 2 months ago
The Andromedarians just like to be contrary.
- cosmoLv 72 months ago
The expansion of the Universe dominates galaxy kinematics on scales larger than about 100 million lightyears. On scales smaller than that, groups of galaxies and galaxy clusters are dominated by their own self-gravity and are bound by their own self-gravity.
- RaymondLv 72 months ago
The Universe? We don't know.
What we know is that SPACE is expanding. As far as we know, space is expanding everywhere (including inside individual photons of light) at, roughly, the same rate.
And that rate is (locally) quite slow. Over a distance of a million parsecs (3.26 million light-years), the rate is approximately 70 km per second (a little under 44 US miles per second). Considering the distance represented by a million parsecs, this is an extremely small amount of "new space" being added over the distance, in a second.
Over extremely long distances, it adds up. For example, at a distance of 100 million parsecs, the distance increases by 100 * 70 = 7,000 km every second. At the limit, at a distance of almost 4300 million parsecs (roughly 13.8 billion light-years), the amount of new space being added every second would be equivalent to the speed of light.
A photon emitted by an object at that distance, and coming straight for us, would see the amount of space between it and us, increases by exactly the same rate as its speed. Conclusion: it never gets here. No wonder 13.8 billion light-years is the limit of the Observable Universe.
If the rate of expansion were to remain constant, that limit would never change (therefore, the Observable Universe would not expand, even though it is "full" of expanding space).
The Andromeda galaxy belongs to the same group of galaxies (the Local Group) as our own Galaxy. All objects within the group are gravitationally bound to each other and they move, in irregular orbits, around the barycentre (centre of mass) of the Group. Unlike the Solar system, the Local Group does not have a central mass; it consists of three large spiral galaxies and forty dwarf galaxies (and other "stuff").
Andromeda is only 2.5 million light-years from us (three-quarters of a million parsecs) so that the space between us (if both galaxies were at rest relative to their local space) would be expanding by only 53 km/s. This is nothing, compared to the actual proper motion of each galaxy (we know our own Galaxy is moving at 400 km/s relative to the Cosmological Microwave Background radiation).
Expansion of space cannot prevent the collision.
An exagerated comparision would be like wondering why the expansion of space does not prevent a pitched baseball from reaching home plate. At that scale, the rate of expansion is a lot less than the size of one proton per second.